The present invention relates to stable topical cosmetic/pharmaceutical emulsion compositions containing ascorbic acid. More particularly, the invention relates to topical emulsion compositions containing ascorbic acid (vitamin C) which are stabilized by means of certain clay/cationic surfactant combinations.
Considerable effort has been expended to find ways to prevent adverse changes in the skin brought about by environmental conditions such as ultraviolet (UV) exposure. Ascorbic acid has many known biological functions from enzymatic co-factor to "sparing" agent against vitamin E depletion. The latter function may partly account for its "anti-oxidant" status. Additionally, at higher concentrations, ascorbic acid is known to react with both superoxide and hydroxyl radicals. Superoxide and the subsequently generated hydrogen peroxide and hydroxyl radical are oxygen-containing free radicals now known to be generated in vivo under a variety of normal and pathological conditions. Quite simply, these radicals have been implicated as causative agents for everything from sunburn to aging. These radicals destroy lipid membranes, break down DNA, inactivate enzymes and so forth. An immense amount of work has been done in the last two decades documenting the deleterious behavior of oxygen radicals.
L-Ascorbic acid (or vitamin C) is chemically defined as an .alpha.-keto-lactone with the following structure: ##STR1## The number 2 and 3 carbons are double-bonded and contain an acid-ionizable hydrogen in water (pK=4.2). Ascorbic acid is also a moderately strong reductant. These properties, which lead to instability in the ascorbic acid structure, are well known and have been burdensome to pharmacologists when attempting to formulate active ascorbic acid solutions. Thus, at higher pH's, the ascorbic acid increasingly becomes the notoriously unstable ascorbate anion. This instability may be due to several causes not restricted to:
a) Stereochemical strain due to polar repulsive forces. Thus, when the 2-hydroxy group ionizes, it places two negative charges (the other being on the carboxyl oxygen) in close proximity which favors ring disruption. PA1 b) Oxidative degradation due to the ascorbate anion's propensity to act as a reductant. The one-electron oxidation product (dehydroascorbate free radical) tends to disproportionate, forming another ascorbate molecule and the two-electron oxidation product, dehydroascorbate, which is extremely unstable in aqueous solution and breaks down to ultimately form species such as L-threonic acid and oxalic acid. Transition metal ions can catalyze these reactions. PA1 c) Degradation due to water attack. At lower ascorbic concentrations or ionic strength, water itself can react with and degrade the ascorbate molecule.
For these reasons, among others, scientists working in the field have had difficulty in formulating stable solutions of ascorbic acid which would be useful for cosmetic or dermatological needs. Nevertheless, because of the many beneficial pharmaceutical effects attributed to ascorbic acid, numerous attempts have been made to overcome these difficulties.
Thus, the literature describes ascorbic acid compositions formed by using a very low weight percent ascorbic acid, or a nonaqueous solvent, or by using derivatives of ascorbic acid, usually in a solution buffered to a pH above 4.0. Also, see, for example, Takashima et al, "Ascorbic Acid Esters and Skin Pigmentation," Am. Perfumer & Cosmetics 86: 29 (July 1971) (esterifying the hydroxyl group to form ascorbic acid-3-phosphate and maintaining an alkaline pH); Ciminera and Wilcox, "Stable Ascorbic Acid Solution for Parenteral Use", J. Am. Pharm. Assoc. Sci. Ed. 35: 363 (1946) (buffering an aqueous solution with an alkaline sodium salt). See also U.S. Pat. No. 4,367,157 which discloses stabilizing an aqueous ascorbic acid solution by adding monothioglycerol and maintaining the pH between 4 and 7; U.S. Pat. No. 2,400,171 which discloses stabilizing ascorbic acid by converting it to its calcium or zinc salt and preferably maintaining the pH at 7 to 7.3; U.S. Pat. No. 2,442,461 which discloses stabilizing calcium ascorbate by adding an aliphatic thiocarboxylic acid and maintaining the pH between 5.2 and 5.6; U.S. Pat. No. 2,585,580 which discloses stabilizing ascorbic acid with thio-sugars and maintaining the pH between 4.0 and 6.5; and U.S. Pat. No. 4,372,874 which discloses actually removing the water to below 0.5 wt. % by using a desiccant. In many cases, these techniques have been successful in obtaining stable solutions but have been reasonably expensive and have yielded a product with less desirable properties than ascorbic acid in its unmodified form.
In addition, many of the cosmetic and dermatological topical formulations are in the form of emulsions. In addition to the known instability of ascorbic acid in water, the acidity introduced by the ascorbic acid has the effect of breaking down emulsions almost immediately at elevated temperature and over a few days at room temperature.
One method of overcoming the tendency of the ascorbic acid to break emulsion is by the use of a carrier containing a hydrophilic gelling agent in the water phase of the emulsion. The product is designed so that upon use, the ascorbic acid powder is added to an aqueous carrier containing the hydrophilic gelling agent and the product is shaken to mix the two. The hydrophilic gelling agents used generically include polysaccharides, synthetic polymers, and celluloses, e.g., carrageenan, guar, xanthan, celluloses such as hydroxyalkylcellulose and sodium carboxycellulose, gelatin, agar--agar, and amidon. This is disclosed in EP 0 679 387.